The research and development in the region of organic photoactive devices, formed in a known embodiment as an organic solar cell or organic photovoltaic cell, has sharply increased in the last ten years. The maximal degree of efficiency previously reported is approximately 5.7% (cf. Jiangeng Xue et al., Appl. Phys. Lett. 85 (2004) 5757). In this manner previously typical efficiencies of 10% to 20% such as are known for inorganic solar cells have not yet been able to be achieved. However, similar results should be achievable with organic solar cells as for solar cells based on inorganic materials.
The advantages of organic solar cells over inorganic solar cells reside in particular in the lower costs. The organic semiconductor materials used are very economical when manufacturing in rather large amounts. A further advantage is formed by the partially very high optical absorption coefficients of up to 2×105 cm−1, which offers the possibility of manufacturing very thin but efficient solar cells with a low expense for material and energy. Since no high temperatures are required in the manufacturing process, namely, substrate temperatures of maximally only approximately 110° C., it is possible to manufacture flexible large-surface structural parts on plastic foil or plastic tissue. This opens up new regions of application that remain closed to the conventional solar cells. On account of the almost unlimited number of different organic compounds, the materials can be tailor-made for their particular task.
In an organic photoactive device light energy is converted into electrical energy. In contrast to inorganic solar cells, in the organic semiconductor material of the organic photoactive devices, the charge carrier pairs (electron-hole pair) are not freely present after the absorption of light but rather they form a quasi-particle, a so-called exciton, namely, a bound electron-hole pair on account of a less strong sheelding of the mutual attraction. In order to make the present energy useful as electrical energy, the exciton formed in this manner must be separated into free charge carriers, that is, an electron and a hole.
Since there are not sufficiently high fields for the separation of excitons in organic solar cells, the separation of excitons is completed on photoactive interfaces. The photoactive interface can be formed as an organic donor-acceptor interface (cf. C. W. Tang, Appl. Phys. Lett., 48 (2), 183-185 (1986)) or as a interface to an inner organic semiconductor (cf. B. O'Regan et al., Nature 1991, 353, 73). The free charge carriers can be transported to the contacts after the separation. The electrical energy can be used by connecting the contacts via a consumer.
An organic material is designated in the sense of the present application as “hole-conducting” if the charge carriers in the material, that are formed as a consequence of light absorption and charge separation on a heterojunction (“photo-generated charge carriers”), are transported in the form of holes. In an analogous manner an organic material is designated as “electron-conducting” if photo-generated charge carriers are transported in the material in the form of electrons. An interface region between the electron-conducting and the hole-conducting material is designated as heterojunction.
A heterojunction between the electron-conducting and the hole-conducting material is designated as a photoactive heterojunction if excitation states that are formed in the electron-conducting and/or the hole-conducting material by the absorption of light and in which charge carriers are bound and that are also called excitons are separated in the region of the heterojunction into the individual charge carriers, namely, electrodes and holes, that for their part are then transported by the electron-conducting material/the hole-conducting material to contacts where electrical energy can be extracted.
A heterojunction between the electron-conducting and the hole-conducting material is designated as a flat heterojunction if the interface between the electron-conducting and the hole-conducting material is formed as a substantially cohesive surface between the two material regions, namely, an region of the electron-conducting material and an region of the hole-conducting material (cf. C. W. Tang, Appl. Phys. Lett., 48 (2), 183-185 (1986) or N. Karl et al., Mol. Cryst. Liq. Cryst., 252, 243-258 (1994)).
A heterojunction between the electron-conducting and the hole-conducting material is a bulk heterojunction if the electron-conducting material and the hole-conducting material are mixed with one another at least partially so that the interface between the electron-conducting and the hole-conducting material comprises a plurality of interface sections distributed over the volume of the material mixture (cf., e.g., C. J. Brabec et al., Adv. Funct. Mater. 11 (1), 15 (2001)).
Ideally, materials of photoactive layers in organic photoactive devices have a high absorption coefficient in the broadest possible wavelength range, which is coordinated with the solar spectrum. The exciton generated by absorption in the semiconductor material should be able to defuse without great energy losses to the photoactive heterojunction, during which an occurring Stokes shift should be as small as possible. Long exciton diffusion lengths make it possible to maximize the thickness of the organic layers in which absorbed light contributes to the photon flow and thus to further improve the efficiency of the organic photoactive device.
Furthermore, a highest occupied energy level (HOMO) and a lowest unoccupied energy level (LUMO) of the organic acceptor material (electron-conducting material) and of the organic donor material (hole-conducting material) are preferably to be selected in such a manner that on the one hand an efficient separation of the excitons in electrons on the acceptor material and of holes on the donor material takes place, and on the other hand the free energy of the system of generated electron and hole is as large as possible. The latter results in a maximizing of the open circuit photovoltage of the device. The charge carriers should be rapidly separated from each other spatially. Good electron transport on the acceptor material and good hole transport on donor material ensure low losses and result in a good fill factor of the current-voltage characteristics of the organic photoactive device.
Organic solar cells are known in various embodiments from the state of the art:                One contact metal has a large and the other contact metal has a small work function, so that a Schottky barrier is formed with the organic layer (cf. U.S. Pat. No. 4,127,738).        The photoactive layer consists of an organic semiconductor in a gel or a binder (U.S. Pat. No. 3,844,843; U.S. Pat. No. 3,900,945; U.S. Pat. No. 4,175,981 and U.S. Pat. No. 4,175,982).        A charge carrier transport layer is formed that contains small particles with a size of 0.01 to 50 μm that assume the charge carrier transport (cf. U.S. Pat. No. 5,965,063).        A layer of the solar cell contains two or more types of organic pigments with different spectral characteristics (cf. JP 04024970).        A layer of the solar cell contains a pigment that produces charge carriers, and additionally a material that removes the charge carriers (cf. JP 07142751).        Polymer-based solar cells were manufactured containing carbon particles as electron acceptors (cf. U.S. Pat. No. 5,986,206).        A doping of mixed systems was provided to improve the transport properties in multi-layer solar cell (cf. DE 102 09 789).        Arrangement of individual solar cells on top of each other (tandem cell) was formed (U.S. Pat. No. 4,461,992; U.S. Pat. No. 6,198,091 and U.S. Pat. No. 6,198,092). Tandem cells can be further improved by using p-i-n structures with doped transport layers with a large band gap (cf. DE 103 13 232).        
The doping of organic materials is known from document U.S. Pat. No. 5,093,698. The admixture of a doping substance, namely, a substance with high electron affinity for p-doping or of a substance with low ionizing energy for n-doping elevates the equilibrium charge carrier concentration in the doped layer and increases the conductivity. In the state of the art in document U.S. Pat. No. 5,093,698 the doped layers are used as injection layers on the interface to the contacts in electroluminescent devices.